The goal of Project 4, in conjunction with work in the other Projects, is to better understand the biological basis of drug addiction, and to identify major biological targets for potential therapeutic intervention to promote abstinence. Previous studies have shown that chronic cocaine increases the transcriptional activity of CREB in the nucleus accumbens (NAc), leading to induction of the neurotrophic factor BDNF. Our current studies suggest that both CREB and BDNF contribute to addiction-related changes in cocaine self-administration, and the propensity for relapse in long-term withdrawal. However, the NAc contains two major cell types that differentially express dopamine receptor subtypes and opioid peptides, and project to different brain regions. Therefore, studies in Aim I of the Project will utilize state-of-the-art transgenic and viral vector technology, in conjunction with the Transgenic Core, to study the functional role of CREB activity in D1 vs D2 receptorcontaining NAc neurons in these addiction-related changes in cocaine self-administration and relapse behaviors. Previous studies also suggest that CREB activation of BDNF is enabled by epigenetic mechanisms involving acetylation of specific histone residues that regulate promoter activity on the BDNF gene. Thus, Aim II of the renewal will study (with Project 3) regulation of histone acetylation in self-administering animals, including regulation of histone acetyl transferase (HAT) and histone deacetylase (HDAC) enzymes. The functional role of histone acetylation in addiction-like changes in cocaine self-administration and relapse will be studied using viral-mediated overexpression of HAT's and HDAC's to up- and downregulate histone acetylation in NAc neurons. Experiments will also determine whether behavioral changes are dependent on AFosB, another drug-regulated transcription factor of great interest to this PPG, which has been shown to alter histone acetylation in the NAc and to promote motivation for cocaine. We found that dynamic BDNF induction and release during cocaine self-administration activates TrkB receptors in NAc neurons to cause an escalation in cocaine intake and increased propensity for relapse. These long-term behavioral changes could involve TrkB receptor signaling through phospholipase C gamma (PLCy) that regulates activity-dependent neuroplasticity, or TrkB signaling through PI3-kinase or ERK kinase which regulate dendritic branching and spine formation. Therefore, studies in Aim III will determine the role of each TrkB signaling pathway in escalating cocaine intake and the propensity for relapse using TrkB receptor docking mutants that selectively allow coupling to either PLC y or PI3-kinase/ERK kinase signaling. Together, these studies will determine important functional links between CREB/AFosB/histone-regulated gene expression in discrete NAc cell types, BDNF induction, and pathway-specific TrkB receptor signaling in the development of primary behavioral pathologies that underlie cocaine addiction.